The present invention relates to a system for determining the heading and/or attitude of a body moving or stationary, such as an aircraft or a vessel.
There is being developed an attitude determining system (ADS) which receives radio waves transmitted from a plurality of position-fixing satellites by a plurality of antennas disposed at specific positions of a body with one of the antennas being used as a reference antenna, measures the phases of carrier signals of the radio waves, determines the relative geometric relationship of the onboard antennas by determining the relative positions of the antennas with respect to the reference antenna, and thereby measures the three-dimensional attitude of the body.
Another example of a conventional system for determining the attitude of a body utilizes an inertial measurement unit (IMU) sensor like a rate gyro which detects angular velocity.
The conventional ADS determines integer ambiguities involved in a double difference of carrier phase or in a single difference of carrier phase by observing the double difference of carrier phase or the single difference of carrier phase between the carrier phases of the radio waves transmitted from the position-fixing satellites to determine the positions of the antennas relative to the reference antenna. In the conventional integer ambiguity solution method, however, there arise errors in solution statistically and stochastically with the probability of finding a correct integer ambiguity solution being 85% to 95%, for example, wherein the time required for obtaining the integer ambiguity solution greatly depends on observation noise level and baseline lengths (i.e., the lengths between the reference antenna and the other antennas whose relative positions should be determined). Under an ordinary noise level, 30 seconds to a few minutes are required on average to obtain an integer ambiguity solution when the baseline lengths are approximately 1 m. When the noise level is high, it has occasionally be impossible to obtain an integer ambiguity solution even after 10 minutes of observation, for example.
In practical situations, it becomes necessary to obtain integer ambiguity solutions when the combination of satellites used for position fix is altered or when a cycle slip (a situation in which information on integer components of a value in a counter for counting the carrier phases is lost due to inability of a coherent delay lock loop of a carrier reconstruction circuit to track) occurs due to noise, for example. Therefore, if the time for determining the integer ambiguity solution is long, an attitude output for a user would be frequently interrupted for impermissibly long periods of time. Also, if the rate of correct integer ambiguity solutions is low, the system would output unreliable attitude information. This has been a major bottleneck for practical application of the conventional ADS.
In an attitude determining system using the aforementioned IMU sensor, on the other hand, a major problem has been the accumulation of errors which occurs due to the fact that the system determines the three-dimensional attitude of a body by integrating its angular velocity components in the direction of each of three axes x, y, z.
Previous efforts to overcome the aforementioned problems by integrating the conventional ADS and IMU system are found in U.S. Pat. No. 5,349,531, in which attitude information is interpolated with attitude determination results obtained from an output of an IMU sensor when an attitude data output from the ADS is interrupted. The attitude determining system of U.S. Pat. No. 4,754,280 basically determines the attitude of a body based on the output of the IMU sensor and outputs it as attitude information and utilizes the attitude determination results obtained by the IMU system just for estimating and removing IMU sensor errors.
When a widely available low-cost IMU sensor is used in a system for determining the attitude of a body by means of the IMU sensor, however, significant errors would occur, making it impossible to perform accurate error modeling. Therefore, even if the IMU sensor error is estimated and compensated for, it can not be completely removed. Thus, when a maximum permissible error is about 1xc2x0, backup time ensured by the IMU is approximately 30 to 60 seconds at the maximum. This means that the ADS should restore an reliable attitude data output within this backup time, so that the conventional ADS/IMU integrated system has not been suited for practical use.
Also, neither of the aforementioned U.S. Patents are intended to solve the aforementioned problems involved in obtaining an integer ambiguity solution of a single difference of carrier phase or a double difference of carrier phase in the ADS.
In view of the foregoing, it is an object of the invention to provide an attitude determining system which can reliably determine an integer ambiguity solution in a shorter time using an output of an IMU sensor.
Another object of the invention is to provide a measuring system which can obtain an integer ambiguity based on an output of an IMU sensor.
The present invention makes it possible to determine integer ambiguities of single differences of carrier phase or double differences of carrier phase observed when determining relative positions of multiple antennas based on attitude angles of a body obtained by means of an IMU sensor to thereby shorten the time required for obtaining an integer ambiguity solution, so that attitude determining results can be continuously output to a user even if cycle slips frequently occur.
According to one aspect of the present invention, a system for determining the heading and/or attitude of a body comprises at least three antennas which receive radio waves from a plurality of position-fixing satellites with one of the antennas being used as a reference antenna, the antennas being fixed at different positions of the body, means for determining the relative positions of the other antennas with respect to the reference antenna by observing single differences of carrier phase or double differences of carrier phase between carrier phases of the radio waves and by calculating integer ambiguities of the phase differences to thereby determine the heading and/or attitude of the body, an IMU sensor installed on the body, means for obtaining attitude angles of the body based on an output of the IMU sensor, and means for determining the integer ambiguities based on the attitude angles obtained when the integer ambiguities are to be redetermined.
According to another aspect of the invention, a system for determining the heading and/or attitude of a body as described above has the said means for determining the integer ambiguities which comprises means for obtaining a baseline vector in the local coordinate system from the attitude angles obtained based on the output of the IMU sensor, means for obtaining the path difference with respect to the baseline vector, and means for obtaining the integer ambiguities based on the path difference with respect to the baseline vector and measured phase difference.
According to another aspect of the invention, a system for determining the heading and/or attitude of a body comprises two antennas which receive radio waves from a plurality of position-fixing satellites, the antennas being fixed at different positions of the body with one of the antennas being used as a reference antenna, means for determining the relative positions of the other antennas with respect to the reference antenna by observing single differences of carrier phase or double differences of carrier phase between carrier phases of the radio waves and by calculating integer ambiguities to thereby determine the heading and/or attitude of the body, an IMU sensor installed on the body, means for obtaining attitude angles of the body based on an output of the IMU sensor, and means for determining the integer ambiguities based on the attitude angles obtained from the output of the IMU sensor when the integer ambiguities are to be redetermined.
According to further aspect of the invention, a system for determining the heading and/or attitude of a body comprises a plurality of antennas which receive radio waves from a plurality of position-fixing satellites, the antennas being fixed at different positions of the body with one of the antennas being used as a reference antenna, means for determining the relative positions of the other antennas with respect to the reference antenna based on integer ambiguities of the phase differences to thereby determine the heading and/or attitude of the body, an IMU sensor installed on the body, means for obtaining attitude angles of the body from an output of the IMU sensor, and means for determining the integer ambiguities based on the attitude angles obtained from the output of the IMU sensor when the integer ambiguities are to be redetermined.
Since the integer ambiguities of the single differences of carrier phase or the double differences of carrier phase observed when calculating the relative positions of the multiple antennas are directly determined from the attitude angles of the body obtained by means of the IMU sensor in this invention, it is possible to output attitude information to the user substantially without interruption even when the combination of satellites to be used is altered or a cycle slip has occurred. Furthermore, it becomes possible to output highly reliable attitude information because the rate of correct integer ambiguity solutions increases.